The present invention relates to machines used for die cutting thermoformed articles from sheets of thermoformable source material, and specifically to a cutting apparatus for making incisions such as cuts or score lines which are in alignment with vertical sides of the thermoformed articles.
In a typical thermoforming operation, a sheet of plastic thermoformable material is drawn in a web from a roll to a first station where it is heated, and then to a thermoforming die station including a set of thermoforming dies. In the operation, the sheet is moved from one station to another by a conveyor system including a pair of generally parallel chain belts, as are known in the art.
The die set includes the male member, which has a plurality of vertically projecting formations and shapes consisting of the desired final configuration of the thermoformed article, and the female member, which is basically a "negative" of the male member. The male and female dies are moveable in a vertically reciprocable fashion with the plastic sheet sandwiched between the two dies. As the dies come together, the heated plastic softens and forms around the vertically projecting formations and shapes of the male die. The female die assists the formation of the plastic about the male die. In some cases, the male die is equipped with a vacuum assist and the female die with a pressure assist to more effectively draw and shape the plastic about the formations of the male die.
After forming, the male and female dies separate, leaving the sheet with at least one thermoformed article. The number of articles formed is determined by the number of cavities in the die set. After the forming process, the conveyor system is designed to index the sheet as it cools. This indexing moves the recently formed articles a specified distance away from the die set, and makes room at the die set for the next thermoforming cycle, during which a second portion of the sheet of plastic material is shaped by the dies. Through indexing, the use of the material is maximized.
After two or three indexes or thermoforming cycles, the first set of molded articles reaches a knife or die cutting press station, where specially designed knives mounted to depend from an underside of a press block are pressed downward upon the sheet. This downward pressing action cuts the articles from the sheet. In some cases, instead of cutting the articles, the knives are configured to merely score the plastic around the articles, to create hinged seams.
A common problem of such thermoforming operations is that the indexing is imprecise, and the knives improperly cut or even damage the thermoformed articles, creating wasted material. One attempted solution to this problem has been the mounting of the knife to the support block so that the knives are movable in a direction parallel to the direction of travel of the articles on the thermoforming apparatus. These so-called "floating" knives accommodate a certain amount of error in the indexing process, and are normally accurate to .+-. 0.030 inches. However, in some thermoformed articles, this degree of error is unsatisfactorily high.
Another measure commonly taken to avoid misalignment of the thermoformed articles relative to the knives is to shape the articles to have a drafted forward surface which guides the knives to the proper cutting position. These measures are satisfactory for most thermoformed articles, but not for those articles used in packages where the longitudinal edges of the article are not cut by the die press, but instead are merely scored to form hinged flaps. Upon scoring, these hinged flaps are folded backward to support products carried behind the thermoformed article when the article and products are eventually enclosed in a container or box.
In such packages, the score lines need to be located in substantially flush relationship with the corresponding sidewall of the thermoformed article. Conventional floating knives have proved incapable of accurately and reliably making the required score lines in the thermoformed articles.
Consequently, it is an object of the present invention to provide an apparatus for making accurate cuts in thermoformed articles.
It is another object of the present invention to provide an apparatus for accurately cutting thermoformed articles which accommodates misalignment of the thermoformed articles due to inaccurate indexing.
It is still another object of the present invention to provide an apparatus for accurately cutting thermoformed articles which is capable of reliably making score lines in the articles which are substantially flush with the corresponding sidewall of the article.